1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire wherein a carcass ply cord is a continuous cord successively repeating roundtrip between a pair of bead portions in a circumferential direction of the tire. Particularly, the invention relates to a pneumatic radial tire capable of conducting automatic building in the production of an uncured tire and having an excellent bead portion durability.
2. Description of Related Art
In the carcass ply of the general-purpose pneumatic radial tire, it is well-known that organic fiber cords such as nylon cord, polyester cord and the like or inorganic fiber cords such as carbon fiber cord, steel cord and the like are used as a reinforcing member for a pair of bead portions, a pair of sidewall portions and a tread portion.
The carcass ply comprised of a radial arrangement of the above cords has a turnup portion wound around a bead core embedded in the bead portion from an inside of the tire toward an outside thereof, wherein cut ends of the cords are located in an upper end of the turnup portion. During the running of the tire having such a radial carcass ply under loading, a large shearing strain is applied to the cord cut end of the turnup portion and hence the upper end part of the turnup portion results in the occurrence of separation failure.
On the other hand, the tire having the above carcass ply structure requires man power in the building of an uncured tire, which is out of the trend for labor saving and power saving and obstructs the building automation toward a near future.
At present, there are proposed pneumatic radial tires having a carcass ply structure suitable for automatic building wherein cut ends of the cords are not formed in the upper end of the turnup portion, or tire production methods suitable for automatic building. For example, JP-A-6-171306 proposes a carcass fixing method wherein a cord of a radial carcass ply is a round trip arrangement of a continuous cord extending between a pair of bead portions and a return portion of the round trip cord is interposed from both sides thereof in an axial direction of the tire between a pair of bead cores through hard rubber layers having a Shore A hardness of not less than 70.
And also, JP-A-9-155991 is concerned with the same method of producing a pneumatic radial tire as in the above publication in a point that the cord of the radial carcass ply is a round trip arrangement of a continuous cord extending between a pair of bead portions, but proposes a method wherein a return portion of a round trip cord is wound each bead core between a pair of bead cores from an inside of the tire toward an outside thereof in the radial direction likewise the conventional tire and the bead core is constructed with cords as well as a tire manufactured by this method.
According to the methods proposed by the above two publications, the automatic building is certainly facilitated, and the resulting pneumatic tires are superior to the conventional tire in a point that cut ends of the carcass ply cords are not existent in the bead portion.
In the pneumatic tire proposed in the former publication, however, since the return portion of the round trip cord is merely anchored in the bead core through the hard rubber, a phenomenon of pulling out the carcass ply cord from the bead core is caused as the running of the tire proceeds under an inflation of an internal pressure and hence a sufficient bead portion durability can not be obtained.
On the other hand, in the pneumatic tire proposed in the latter publication, the roundtrip return portion of the carcass ply cord forms a turnup portion around the bead core, so that the above inconvenience of the former publication may be improved to a certain extent, but since the bead core is constructed with the cords or twisted wires and a single bead core is used in each bead portion, the elongation is large along a circumference of the bead portion and the bead portion stiffness is lack and hence the development of the phenomenon of pulling out the carcass ply cord is accompanied and there is still left a problem that the bead portion durability is lacking.